if ( my (@caps) = ($line =~ $re) ) {
    @caps = () if $caps[0] ne $1;           # reset pseudo captures
    $cb->(@caps);
    last;
}
[download]

In
    @caps = () if $caps[0] ne $1;
$1 will be undefined if there is no capture group 1 or it is something like (...)? that fails to match (but an overall match can still occur). Warnings will be generated.

Although it's only compatible back to version 5.6, an alternative would be

c:\@Work\Perl\monks>perl

use strict;
use warnings;

use Data::Dump qw(pp);

my $cb = sub { print "matched,  captured ", pp @_; };

for my $str ("AB","") {

    for my $re (qr/../, qr/(.)(.)/, qr/(X)?(.)/, qr/XY/, qr/(X)Y/, qr/
+/) {

        print "str <$str>  re $re  ";

        if ( my (@caps) = ($str =~ $re) ) {
          # @caps = () if $caps[0] ne $1;  # reset pseudo capture
            $#caps = $#- - 1;  # reset pseudo capture
            $cb->(@caps);
            }
        else {
            print 'NO match';
            }

        print "\n";

        }

    }

^Z
str <AB>  re (?-xism:..)  matched,  captured ()
str <AB>  re (?-xism:(.)(.))  matched,  captured ("A", "B")
str <AB>  re (?-xism:(X)?(.))  matched,  captured (undef, "A")
str <AB>  re (?-xism:XY)  NO match
str <AB>  re (?-xism:(X)Y)  NO match
str <AB>  re (?-xism:)  matched,  captured ()
str <>  re (?-xism:..)  NO match
str <>  re (?-xism:(.)(.))  NO match
str <>  re (?-xism:(X)?(.))  NO match
str <>  re (?-xism:XY)  NO match
str <>  re (?-xism:(X)Y)  NO match
str <>  re (?-xism:)  matched,  captured ()
[download]

Update: But see haukex's reply about the preference for $#+ versus $#- (@+ versus @-) regex special variables.

$#caps = $#- - 1;

As I linked to in my update here, there is a subtle but important distinction between $#- and $#+. You can see this in action if you add e.g. qr/A(X)?/ as one of the test regexes, with $#+ - 1 the resulting @caps will be (undef) instead of being empty, thus correctly reflecting the number of capture groups present in the regex. Yet another solution might be $cb->($#+ ? @caps : ()).

> $1 will be undefined if there is no capture group 1

I was aware of that and my results are exactly like yours.

In the following I just need to silence the warnings about undef values.

use v5.12;
use warnings;

use Data::Dump qw(pp);


my $cb = sub { say "matched,  captured ", pp @_; };

for my $str ("AB","") {

    for my $re (qr/../, qr/(.)(.)/, qr/(X)?(.)/, qr/XY/, qr/(X)Y/, qr/
+/) {

        say "--- str <$str>  re $re  ";

        if ( my (@caps_0) = ($str =~ $re) ) {

            my @caps = @caps_0;
            $#caps = $#- - 1;           # reset pseudo capture
            $cb->(@caps);

            @caps = @caps_0;
            no warnings 'uninitialized';
            @caps = () if $caps[0] ne $1; # reset pseudo capture
            $cb->(@caps);

        } else {
            say 'NO match';
        }

        say "\n";

    }

}
[download]

--- str <AB>  re (?^u:..)  
matched,  captured ()
matched,  captured ()


--- str <AB>  re (?^u:(.)(.))  
matched,  captured ("A", "B")
matched,  captured ("A", "B")


--- str <AB>  re (?^u:(X)?(.))  
matched,  captured (undef, "A")
matched,  captured (undef, "A")


--- str <AB>  re (?^u:XY)  
NO match


--- str <AB>  re (?^u:(X)Y)  
NO match


--- str <AB>  re (?^u:)  
matched,  captured ()
matched,  captured ()


--- str <>  re (?^u:..)  
NO match


--- str <>  re (?^u:(.)(.))  
NO match


--- str <>  re (?^u:(X)?(.))  
NO match


--- str <>  re (?^u:XY)  
NO match


--- str <>  re (?^u:(X)Y)  
NO match


--- str <>  re (?^u:)  
matched,  captured ()
matched,  captured ()
[download]

Cheers Rolf
_{(addicted to the 𐍀𐌴𐍂𐌻 Programming Language :)
Wikisyntax for the Monastery}